The present invention relates to a method and an apparatus for forming a coating film on a substrate such as a semiconductor wafer and a LCD substrate (a glass plate for liquid-crystal displays) by applying a coating solution to be used for device-protective films, etc.
A procedure of applying polyimide as a coating solution on a substrate such as a semiconductor wafer for forming a protective film or inter-layer insulating film is one of several procedures of fabricating semiconductor devices.
A known method for the coating procedure is as follows: A coating solution is prepared which is made of polyimide dissolved in a solvent. The coating solution is further diluted with another solvent.
The coating solution is then sprayed on a wafer W, as illustrated in FIG. 1. In detail, a coating nozzle 11 travels over the wafer W little by little and almost horizontally in the radius direction while the wafer W is rotating. The coating solution is spirally splayed on the rotating wafer W from the nozzle 11 with a single stroke. The wafer W is then heated to evaporate the solvent and cooled to form a coating film thereon.
The known method includes two techniques: (1) The coating solution is sprayed on the wafer W at a constant supplying rate so that lines of the coating solution sprayed as adjacent on the wafer W will be arranged tight or with a uniform gap therebetween, for forming a coating film with a high uniformity in thickness over the wafer. (2) The nozzle 11 is controlled with a constant spraying amount for the coating solution so that it will travel at a high speed over the center of the wafer but at gradually slower speeds towards the edge, for forming a coating film with a high unifonnity in thickness over the wafer surface.
The technique (1), however, has the following drawbacks: A coating film formed by this technique will exhibit a high uniformity in thickness over the wafer surface, but will not be round perfectly when viewed from above, thus stress being applied irregularly over the entire film surface.
For example, it is assumed that the nozzle 11 stops when the outer edge of a line of coating solution touches an area to be coated P indicated by a dot-line circle in FIG. 2.
This results in that the outermost line of coating solution Q1 will not become a perfect circle because it is drawn spirally, thus a bump of the coating film, or a significant section (area) will be formed at an edge Q2, with a small outer area R which will not be coated.
The supply of coating solution from the nozzle 11 may not necessarily be brought to a halt immediately by a user operation. The nozzle 11 mostly has a small diameter such as 100 m in actual use, so that a coating solution passing through the nozzle could easily suffer a residual pressure in the nozzle. This could result in that an excess coating solution drips from the nozzle and over-supplied to the wafer surface after the supply of coating solution is brought to a halt. Such an excess coating solution could be over-supplied to the edge Q2 (FIG. 2) so that the coating film will be formed at this area thicker than the other areas on the wafer W, as the bump, or the significant section (area).
Generation of such a significant section on a coating film could cause irregularity in stress over the coating-film surface in subsequent procedures such as drying under reduced pressure, cooling and heating. This results in variation in degree of treatment on the coating-film surface by these procedures. For example, the evaporation of solvent will not be constant over the coating-film surface in the drying procedure under reduced pressure, thus causing irregularity in coating-film thickness.
The irregularity in film thickness could further pose problems on developing and etching, the subsequent procedures. For instance, a photosensitive polyimide could change developing performance and thus form no holes on the semiconductor wafer whereas a non-photosensitive polyimide could cause variation in etching-proof performance over the coating-film surface thus cause un-etched parts on thick film areas.
The technique (2) also has the following drawbacks:
As shown in FIG. 3, a coating solution 12 including polyimid was sprayed on a 8-inch wafer W to form an about 10 μm-thick coating film while the speed of the nozzle 11 is varying. It was observed that bumps were formed at the outermost edges of the wafer, about 0.5 μm thicker than the other areas, after heating and drying.
Such thick parts at the outermost edges could also affect developing and etching performance and thus form no holes, or un-etched areas remain on the wafer.
The following is a possible explanation of these problems.
The solvent in the coating solution 12 is evaporated from the entire film surface and also the side edges of the outer areas of the wafer W. The degree of evaporation at the outer areas of the wafer W is higher than the other areas, thus an evaporating rate of the solvent is also higher at the outer areas than the other areas. The evaporation of solvent from an area at a high evaporating rate causes decrease in temperature and thus increase in surface tension on this area. The increased surface tension pulls the coating solution towards the outer areas, which results in a thick polyimide film formed on the outer areas due to a high solidification rate of polyimide.
In FIG. 3, as discussed above, a high evaporating rate of the solvent on the outer areas could have caused high solidification to polyimide and thus a thick film was formed on the outer areas.